Immune responses are largely mediated by a diverse collection of peripheral blood cells termed leukocytes. The leukocytes include lymphocytes, granulocytes and monocytes. Granulocytes are further subdivided into neutrophils, eosinophils and basophils. Lymphocytes are further subdivided into T and B lymphocytes. T-lymphocytes originate from lymphocytic-committed stem cells of the embryo. Differentiation occurs in the thymus and proceeds through prothymocyte, cortical thymocyte and medullary thymocyte intermediate stages, to produce various types of mature T-cells. These subtypes include CD8.sup.+ T cells (also known as cytotoxic/suppressor T cells), which, when activated, have the capacity to lyse target cells, and CD4.sup.+ T cells (also known as T helper and T inducer cells), which, when activated, have the capacity to stimulate other immune system cell types.
Immune system responses are elicited in several differing situations. The most frequent response is as a desirable protection against infectious microorganisms. However, undesired immune response can occur following transplantation of foreign tissue, or in an autoimmune disease, in which one of a body's own antigens is the target for the immune response. Immune responses can also be initiated in vitro by mitogens or antibodies against certain receptors. In each of these situations, an immune response is transduced from a stimulating event via a complex interaction of leukocytic cell types. However, the participating cell types and nature of the interaction between cell types may vary for different stimulating events. For example, immune responses against invading bacteria are often transduced by formation of complexes between an MHC Class II receptor and a bacterial antigen, which then activate CD4.sup.+ T-cells. By contrast, immune responses against viral infections are principally transduced by formation of MHC Class I/viral antigen complexes and subsequent activation of CD8.sup.+ cells.
Over recent years, many leukocyte cell surface antigens have been identified, some of which have been shown to have a role in signal transduction. It has been found that signals may be transduced between a cell-surface receptor and either a soluble ligand or a cell-surface-bound ligand. The amino acid sequences of leukocyte surface molecules comprise a number of characteristic recurring sequences or motifs. These motifs are predicted to be related in evolution, have similar folding patterns and mediate similar types of interactions. A number of superfamilies, including the immunoglobulin and nerve growth factor receptor superfamilies, have been described. Members of the nerve growth factor receptor family include NGFR, found on neural cells; the B-cell antigen CD40; the rat OX-40 antigen, found on activated CD4.sup.+ cells (Mallet et al., EMBO J. 9:1063-1068 (1990) (hereby incorporated by reference for all purposes); two receptors for tumor necrosis factor (TNF), LTNFR-1 and TNFR-II, found on a variety of cell types; 4-1BB found on T-cells; SFV-T2, an open reading frame in Shope fibroma virus; and possibly fas, CD27 and CD30. See generally Mallet & Barclay, Immunology Today 12:220-222 (1990) (hereby incorporated by reference for all purposes).
The identification of cell-surface receptors has suggested new agents for suppressing undesirable immune responses such as transplant rejection, autoimmune disease and inflammation. Agents, particularly antibodies, that block receptors of immune cells from binding to soluble molecules or cell-bound receptors can impair immune responses. Ideally, an agent should block only undesired immune responses (e.g., transplant rejection) while leaving a residual capacity to effect desirable responses (e.g., responsive to pathogenic microorganisms). The immunosuppressive action of some agents, for example, antibodies against the CD3 receptor and the IL-2 receptor have already been tested in clinical trials. Although some trials have shown encouraging results, significant problems remain. First, a patient may develop an immune response toward the blocking agent preventing continued immunosuppressive effects unless different agents are available. Second, cells expressing the target antigen may be able to adapt to the presence of the blocking agent by ceasing to express the antigen, while retaining immune functions. In this situation, continued treatment with a single immunosuppressive agent is ineffective. Third, many targets for therapeutic agents are located on more than one leukocyte subtype, with the result that it is generally not possible to selectively block or eliminate the response of only specific cellular subtypes and thereby leave unimpaired a residual immune capacity for combating infectious microorganisms.
Based on the foregoing it is apparent that a need exists for additional and improved agents capable of suppressing immune responses, particularly agents capable of selective suppression. The present invention fulfills these and other needs, in part, by providing a ligand (ACT-4-L) to a receptor localized on activated human CD4.sup.+ T-lymphocytes.